This invention relates generally to lasers and, more specifically, to semiconductor or diode lasers operated in an external cavity.
As noted in the parent application, Ser. No. 07/341,028,now U.S. Pat. No. 5,050,179, numerous advantages accrue to operation of semiconductor lasers in an external cavity as contrasted to solitary diode laser operation. In external cavity-controlled semiconductor lasers, a diode laser with opposed anti-reflective (AR) coated first and second facets is mounted between a pair of AR coated lenses. The facets extend in a plane perpendicular to the plane of the diode gain region extending along the length of the diode laser. One of the lenses collects the laser radiation from the first facet and collimates it onto a frequency tuning element, such as a diffraction grating, where the incident beam is dispersed and reflected colinear with the incident beam and re-imaged onto the first diode facet. Radiation from the second facet is collimated onto an output coupler formed by a dielectric coated, partially reflecting plane mirror.
Selection and tunability of the emission wavelength of the diode laser is achieved by varying the tilt of the diffraction grating to select an appropriate single frequency within the bandwidth of the diode laser to feed back energy to lock the diode laser at the selected frequency. This is in contrast to single diode laser operation which is amenable to reversible frequency control only by externally induced variation of band gap energy and refraction index (through changes in temperature or pressure for example) or by variation of injection current.
Stable, single-mode laser operation in an external cavity necessitates a high degree of thermal and mechanical stability in the cavity. Furthermore, the lens which collimates the laser radiation onto the diffraction grating must be accurately aligned with the facet to avoid mode instabilities. The linear alignment precision requirement is in the order of one wavelength. Alignment is a time-consuming, tedious procedure requiring expensive calibrating equipment. Of necessity, therefore, this alignment process is presently performed by the cavity manufacturer before the cavity is sold to the consumer.
The spectral range of the diffraction grating is in the order of 400 nm, whereas the spectral range of most diodes lasers is only about 50 nm. Therefore, when output wavelengths are required beyond the range of a given diode, as a practical matter, it is usually necessary to purchase another external cavity package with a diode operating in the desired wavelength range to avoid the aforementioned tedious realignment.